1. Technical Field of the Invention
The present invention relates to coil forming methods and coil forming die assemblies and, more particularly, to a coil forming method of conducting preliminary forming work on a coil made of a conductive raw material with an outer circumference covered with a film, and subsequently conducting bending work on the coil, subjected to the preliminary forming work, to form a rounded corner portion and a coil forming die assembly to be sued for carrying out such a coil forming method.
2. Description of the Related Art
In the related art, it has been a general practice to employ a coil, having a rectangular shape in cross section and covered with a film, to be used in a stator or a rotor of an electric rotary machine (see FIG. 10A). When bending work is conducted on the coil 100 at an angle of, for instance, 90 degrees with a rounded corner portion being formed at a bent area, a deformation occurs in a conductive raw material 110 of the coil 100 in an inner root area of the rounded corner portion. When this takes place, an inner body portion 110, facing inward of the coil at the rounded corner portion, has both end portions bulged in a widthwise direction of the coil due to compression strain born in the conductive raw material 110 as shown in FIG. 10B. In this case, an inside circumferential wall 100a of the rounded corner portion is inwardly convexed and an outside circumferential wall of the rounded corner portion is inwardly concaved with sidewalls 100c being inwardly concaved. Especially, bulged portions 110a and 110b of the inner circumferential body portion 110a interfere with adjacent bulged portions of neighboring coil (not shown), causing a reduction in lamination factor. To address such an issue, the bulged portions 110a1 and 110a2 of the inner body portion 110a of the coil 100 are compressed in the widthwise direction. This causes a film 120 to be distorted at areas 120a1 and 120a2 covering the bulged portions 110a1 and 110a2 (at corner areas) of the conductive raw material 110, respectively, to be thinned in thickness, resulting in an issue with the occurrence of a defect in an electrically insulating effect with the adjacent coil. In addition, due to stretching strain caused in the conductive raw material 110, the outer body portion 110b, placed outward of a neutral axis NA (representing a position at which compression strain and stretching strain are zeroed), has the film 120 with a decreased thickness. Thus, a need arises to minimize a reduction in thickness of the film.
To address such an issue, an attempt has been made to conduct bending work on the coil 100 with both sides thereof restricted with a die frame 130 or the like in a manner as shown in FIG. 11 (as disclosed in Japanese Patent no. 3894004).
With the prior art method conducting bending work on the coil 100 with the both sides being restricted with the die frame 130, stress remains in an internal part of the conductive raw material 110 after bending work being conducted. Due to an effect of such internal stress, the coil 100 expands (restores in elastic formation) at areas restricted with the die frame during bending work, resulting in a difficulty of diminishing the bulged portions 110a1 and 110a2 of the inner body portion 110a at the inner root area of the rounded corner portion. Further, with the prior art method described above, it is hard to minimize the reduction in thickness of the film 120 of the coil 100 at the outer body portion 110b thereof.
Furthermore, since bending work is conducted with the both sides of the coil 100 being restricted with the die frame 130, there is a risk that damage will be caused to the film 120 of the coil 100.
With bending work conducted on the coil 100, the outside circumferential wall 100b of the coil 100 at the rounded corner portion thereof has a central area formed in an inwardly concaved shape curved in a saddle-like configuration. Meanwhile, the inside circumferential wall 100a of the coil 100 at the rounded corner portion thereof has both sides lowered in height with a central area bulged to form a convexed shape. With such deformation of the coil 100 in shape, the inner body portion 110a of the coil 100 has the bulged portions 110a1 and 110a2. Thus, the coil 100, deformed in such a structure, has an interference with the adjacent coil, causing an issue to arise with the occurrence of a drop in lamination factor.